Electrical resistance heating elements are used to heat water in a wide variety of applications. For example, heating elements are used in water heaters for water systems in commercial and residential buildings, pools and baths, and spas. The water chemistry environment to which heating elements are exposed depends upon the application. Of the applications listed above, spas and hot-tubs perhaps present the harshest environments to water heating elements, because they recirculate water.
As a result, water chemistry must be monitored frequently for sanitary reasons and for corrosion control. Monitored parameters typically include pH, total alkalinity, calcium hardness, and total dissolved solids. For example, pH is desirably maintained around a neutral pH level of 7.0. If any of these parameters is outside a predetermined band of acceptable values, then corrective action should be taken to return water chemistry parameters to values within the predetermined band. Unfortunately, water chemistry may not be monitored sufficiently, or corrective action may not be taken when warranted.
If water chemistry parameters are allowed to remain outside the predetermined band of acceptable values, corrosion of metallic spa and hot tub components can result. Typical corrosion mechanisms include galvanic corrosion, chemical pitting, intergranular corrosion, stress corrosion cracking, corrosion fatigue, electrochemical corrosion, and bacterial corrosion due to Ferrobacillus bacteria. Corrosion of conventional heating elements is an all too frequent cause of spa breakdown.
One component of a spa heating element that is particularly susceptible to corrosion is a bulkhead flange. In a typical heating element known in the prior art, the heating element is contained within a stainless steel heater housing. The heater housing has an aperture for receiving the ends of the heating element therethrough. The heating element is constructed from a heating coil that has two ends, each connected to a cold pin. The heating coil and the cold pins are coaxially housed within a tubular outer sheath of stainless steel filled with a dielectric material. An annular stainless steel bulkhead flange is typically brazed about the outer sheath adjacent each end of the element. A bulkhead gasket is placed between the bulkhead flange and the interior of the housing. A bulkhead nut is then threaded onto each cold pin so the bulkhead flange and the bulkhead nut hold the bulkhead gasket in sealing engagement with the interior of the housing. Thus, the sheath-to-bulkhead flange braze is completely surrounded by water. Further, the bulkhead flange is surrounded by water, except where the bulkhead flange abuts the bulkhead gasket.
As a result, corrosion of the bulkhead flange and the sheath-to-bulkhead flange braze can, and often do, occur when water chemistry parameters are allowed to remain out of the predetermined band of acceptable values. The bulkhead flange is machined and the outer sheath is bent and formed, so the ingredients in their respective stainless steels differ. Accordingly, galvanic corrosion can result from improper control of water chemistry. Further, care must be taken when brazing the outer sheath to the bulkhead flange to ensure no impurities remain in the brazed joint. Braze impurities, along with out-of-specification water chemistry parameters, can result in corrosion at the braze joint.
The corrosion of the bulkhead flange and the sheath-to-bulkhead flange braze joint can range from a rust-like deposit to cracks and fissures in the metal. The corrosion can even lead to corrosion fatigue, causing the bulkhead flange to break away from the outer sheath. If the bulkhead flange breaks away from the outer sheath, the heating element may be damaged beyond repair. In this case, the heating element must be discarded. This type of failure thus represents a costly problem.
Other attempts have been made to address the shortcomings of brazed bulkheads by welding the element to the housing. However, this results in an element that is not replaceable in the field, and weld failure may still occur due to corrosion.
Rather than brazing a bulkhead onto an element sheath, another conventional style is to weld a bulkhead to a sheath, on the upper (threaded) side of the element. A void between the bulkhead and sheath is then sealed. Wile this protects the weld joint, the bulkhead is still wetted and subject to corrosion.